Sialidase localized in plasma membrane and DNA coding for the same

ABSTRACT

The present invention provides a protein defined in the following (A) or (B), and DNA coding for the same:  
     (A) a protein which has the amino acid sequence of SEQ ID NO: 2 or SEQ ID NO: 4, or  
     (B) a protein which has the amino acid sequence including substitution, deletion, insertion, or transition of one or several amino acid residues in SEQ ID NO: 2 or SEQ ID NO: 4, and exhibits activity to eliminate a sialic acid residue from a non-reducing terminal of ganglioside.

TECHNICAL FIELD

[0001] The present invention provides a novel sialidase and DNA coding for it. More precisely, the present invention provides sialidase that localizes in plasma membrane, and specifically hydrolyzes gangliosides, and DNA coding for it.

[0002] The sialidase of the present invention and the DNA coding for it are expected to be utilized as a reagent used for saccharide chain studies and a medicament used for gene diagnosis and gene therapy.

BACKGROUND ART

[0003] Sialidase is a glycohydrolytic enzyme present in living bodies, which eliminates a sialic acid residue from a non-reducing terminal of saccharide chains of glycoproteins or glycolipids. It has been known that, when sialic acid is removed from saccharide chain molecules, not only the degradation of these molecules begins, but also molecular conformation and many of important cell functions such as recognition mechanism by receptors, cell adhesion, and immunomechanism may be changed. It has also become clear that sialidase exhibits rapid activity change in connection with proliferation and canceration of cells, and it is involved in the metastatic ability of cancer cells. However, there is little knowledge about how sialic acid is eliminated in vivo. This is because the studies of mammalian sialidases on the molecular level are behindhand, and hence there are many unknown points concerning their structures and expression mechanism.

[0004] Because mammalian sialidase exhibits only low activity, and is extremely unstable, isolation and purification of the enzyme have been difficult. Sialidase has been often considered for a long time to be one of the mere lysosomal enzymes involved in the dissimilation and degradation. Under such a situation, we isolated and purified the enzyme by using mainly rat tissues as the source of the enzyme, and found that there were four types of sialidases which differ from sialidases of bacteria, viruses, protozoa and the like in their natures (Miyagi, T. and Tsuiki, S., Eur. J. Biochem. 141, 75-81, 1984; Miyagi, T. et al., J. Biochem. 107, 787-793, 1990; Miyagi, T. and Tsuiki, S., J. Biol. Chem. 260, 6710-6716, 1985). These enzymes each localize in lysosomal matrix, lysosome membrane, plasma membrane (cell surface membrane), and cytoplasm within a cell, respectively, and they are different from each other not only in enzymological characteristics such as substrate specificity, but also in immunological properties. Among those sialidases, the sialidase localized in cytoplasm can be obtained as a homogenous purified product from rat skeletal muscles. Its cDNA cloning has been succeeded for the first time in the world as for animal sialidases, and its primary structure has been determined (Miyagi T. et al., J. Biol. Chem., 268, 26435-26440, 1993). Its genomic structure analysis has also been done, and as for its function, it has been elucidated that the enzyme participates in the differentiation and the growth of skeletal muscle cells by using the cDNA as a probe. These studies can be considered a part of pioneer researches in sialidase studies in the world.

[0005] By the previous studies, it has become clear that there is possibility that the sialidase localized in plasma membrane exhibits activity elevation upon proliferation and canceration of cells, and it is also deeply concerned with the differentiation of nerve cells and the signal transduction of cells. To date, however, it has not been understood at all about the structure of this enzyme, the mechanism causing the activity change and the like. In order to answer these questions, what many researchers in this field have long been desired is cloning of its cDNA. For example, if the mechanism of cancerous change due to this enzyme could be elucidated, it would be possible to utilize the results in diagnosis and therapy of cancers. Further, because gangliosides exist in surface membranes of many cells and participate in important cell functions such as cell adhesion and informational communication, and they are also main cerebral components, the sialidase utilizing them as a specific substrate is estimated to be involved in certain important cranial nerve functions.

SUMMARY OF THE INVENTION

[0006] The present invention has been accomplished in view of the aforementioned present condition. An object of the present invention is to provide the sialidase localized in plasma membrane and DNA that codes for it.

[0007] The inventors of the present invention earnestly conducted studies in order to achieve the aforementioned object, and as a result, succeeded in isolating the sialidase localized in plasma membrane and cloning of cDNA coding for it. Furthermore, they found that the aforementioned sialidase was unique in that it substantially specifically hydrolyzed gangliosides (glycolipids containing sialic acid), which are substrates that similarly localize mainly in plasma membrane, and it was completely different from other mammalian sialidases and microbial sialidases in enzymatic substrate specificity. Thus, they accomplished the present invention.

[0008] That is, the present invention provides a protein defined in the following (A) or (B):

[0009] (A) a protein which has the amino acid sequence of SEQ ID NO: 2 or SEQ ID NO: 4, or

[0010] (B) a protein which has the amino acid sequence including substitution, deletion, insertion, or transition of one or several amino acid residues in SEQ ID NO: 2 or SEQ ID NO: 4, and exhibits activity to eliminate a sialic acid residue from a non-reducing terminal of ganglioside.

[0011] The present invention also provides DNA coding for the protein defined in the above (A) or (B). Specifically, such DNA may be DNA which has the nucleotide sequence of SEQ ID NO: 1 or SEQ ID NO: 3.

[0012] A sialidase which has the aforementioned characteristics will be referred to as the “sialidase of the present invention” hereinafter, and DNA coding for it will be referred to as the “DNA of the present invention” hereinafter.

[0013] Hereafter, the present invention will be explained in detail.

[0014] <1> Sialidase of the Present Invention

[0015] The sialidase of the present invention is a protein which has the amino acid sequence of SEQ ID NO: 2 or SEQ ID NO: 4. Moreover, the sialidase of the present invention include a protein which has the amino acid sequence including substitution, deletion, insertion, or transition of one or several amino acid residues in SEQ ID NO: 2 or SEQ ID NO: 4, so long as it exhibits activity for eliminating a sialic acid residue from a non-reducing terminal of ganglioside.

[0016] Among the aforementioned sialidases, the sialidase that has the amino acid sequence of SEQ ID NO: 2 has the following physicochemical properties.

[0017] (1) Activity

[0018] It eliminates sialic acid residues from a non-reducing terminal of ganglioside.

[0019] (2) Substrate Specificity

[0020] It acts on gangliosides, but not act on glycoproteins and oligosaccharides. Specifically, it acts on GD3-ganglioside, GD1a-ganglioside, GM3-ganglioside, and synthetic gangliosides (GSC-17(α2-3) and GSC-61(α2-6)), but it does not substantially act on GM2-ganglioside, GM1-ganglioside, orosomucoid, fetuin, glycophorin, ovine submaxillary gland mucin, and bovine submaxillary gland mucin. It weakly acts on α2-3 sialyllactose, and 4-MUNeuAc (4-methylumbelliferyl N-acetylneuraminic acid).

[0021] (3) Optimum pH

[0022] 4.7 to 5.0

[0023] (4) Molecular Weight

[0024] About 65,000 as determined by sucrose density gradient centrifugation,

[0025] About 52,000 as determined by SDS-polyacrylamide gel electrophoresis under reducing condition

[0026] (5) Inhibition and Activation

[0027] A surface active agent is required for the activity. For example, it is highly active in the presence of 0.1 to 0.2% of Triton X-100.

[0028] It is strongly inhibited by heavy metal ions such as Cu²⁺, and 4-hydroxy mercury benzoate.

[0029] It is stabilized by dithiothreitol, Neu5Ac2en (2-deoxy-2,3-dehydro-N-acetylneuraminic acid), and glycerol. However, it is weakly inhibited by Neu5Ac2en.

[0030] Among the sialidases of the present invention, the sialidase which has the aforementioned characteristics is an enzyme derived from bovine, whereas the sialidase which has the amino acid sequence of SEQ ID NO: 4 in one derived from human. These exhibit 82% homology in their amino acid sequences, and they have a transmembrane domain, glycosylation site, and Asp-box, which is a consensus sequence of sialidase, at the same locations. Therefore, the enzyme derived from human is considered to have the same physicochemical properties as the enzyme derived from bovine.

[0031] The sialidase of the present invention can be obtained from a bovine brain, for example, as follows. All of the following procedures are preferably carried out at a low temperature.

[0032] A bovine brain is homogenized in a buffer, and centrifuged at 1000×g for 10 minute, and the supernatant is further centrifuged at 30,000×g for 1 hour. After the centrifugation, the precipitate fraction is suspended in a buffer, added 5% deoxycholic acid, sufficiently homogenized, and centrifuged at 100,000×g for one hour to obtain a soluble fraction as a supernatant. The buffer preferably contains an inhibitor for proteases, dithiothreitol, surface active agent and the like.

[0033] The above soluble fraction is applied to a DEAE-cellulose column and, after the column is washed, eluted with a buffer containing 0.2 M NaCl for fractionation. A fraction exhibiting the sialidase activity is dialyzed against a buffer, then applied on Octyl-Sepharose, and separated by elution with a linear gradient of 0.1-0.4% Triton X-100.

[0034] Then, an active fraction is applied to Heparin-Sepharose (Pharmacia), washed with a buffer containing 0.25 M NaCl, and eluted with a 0.2-1 M NaCl linear gradient to concentrate the active fraction. The above concentrated enzyme solution is loaded on Sephacryl S-200 (Pharmacia), and separated by elution with a buffer containing 0.02 mM NeuAc2en (2-deoxy-2,3-dehydro-N-acetylneuraminic acid).

[0035] The obtained active fraction is diluted to have a Triton X-100 concentration of 0.02%, added to RCA-lectin agarose (Pharmacia), washed with a buffer containing 0.02% Triton X-100, and eluted with a buffer containing 0.2 M lactose. This active fraction is loaded on a MonoQ (Pharmacia) column, and eluted with a 0-0.5 M NaCl linear gradient.

[0036] The active fraction is loaded on an activated thiol Sepharose (Pharmacia) column, washed with a 0.15 M NaCl buffer containing 10% glycerol, and then with 0.5 M NaCl buffer containing 10% glycerol, and eluted with 0.05 M NaCl buffer containing 0.05 M dithiothreitol. The active fraction is concentrated in a MonoQ column.

[0037] The above concentrate is loaded on an affinity column utilizing a synthetic ganglioside GM3 [GSC-211, NeuAc-Gal-Glc-O(CH₂)₈NH₂] as a ligand (Hasegawa A. et al. J. Carbohydr. Chem., 9, 201-214, 1990), and separated by elution with a 0-0.5 M NaCl gradient. The affinity column can be obtained by allowing GSC-211 to couple with ECH-Sepharose (Pharmacia) in the presence of N-ethyl-N′-(3′-dimethyl-aminopropyl)carbodiimide hydrochloride.

[0038] The sialidase enzyme is purified as described above as a protein having a molecular weight of 52 kD as determined by SDS-polyacrylamide gel electrophoresis.

[0039] Further, since DNA coding for the sialidase of the present invention has been obtained, the sialidase of the present invention can also be obtained by expressing the DNA in a suitable host-vector system. As for the host-vector system, a cultured cell can be used as a host, and a vector suitable for this host can be used. Materials and methods therefore may be those usually used for the production of heterogenous proteins utilizing genetic recombination techniques. When DNA coding for the sialidase of the present invention is ligated to a vector, a vector containing sequences required for regulation of gene expression such as promoter and terminator that can be expressed in the host as required may be used.

[0040] <2> DNA of the Present Invention

[0041] Because the amino acid sequence of the protein encoded by the DNA of the present invention has been elucidated, the DNA of the present invention can be cloned based on the amino acid sequence. In the examples mentioned below, a partial amino acid sequence of the sialidase of the present invention is determined, oligonucleotide primers are synthesized based on the partial amino acid sequence, and the DNA of the present invention is obtained from a bovine brain cDNA library by PCR (polymerase chain reaction) using the oligonucleotides primers.

[0042] Although the sequence of the DNA of the present invention is not particularly limited so long as it codes for the amino acid sequence of SEQ ID NO: 2 or 4, the nucleotide sequences of SEQ ID NO: 1 and 3 can be specifically mentioned. Further, existence of sialidases having the amino acid sequence of SEQ ID NO: 2 or SEQ ID NO: 4 including substitution, deletion, insertion, or transition of one or several amino acid residues, and genes coding for them is expected due to difference of animal species, individuals or varieties. Such DNA coding for the substantially same protein as the sialidase of the present invention also falls within the scope of the DNA of the present invention. Such DNA can also be obtained from a cell harboring it by hybridizing it with the nucleotide sequence of SEQ ID NO: 1 or 3 or a part thereof under a stringent condition, and isolating DNA coding for a protein which has sialidase activity. DNA coding for a sialidase having such a mutation as mentioned above may also be obtained by, for example, site-specific mutagenesis or mutagenic treatment.

[0043] The term “one or several” amino acid residues means 1-80, preferably 1-30, more preferably 1-5 amino acid residues.

[0044] <3> Pregressive Applications of the Sialidase of the Present Invention and DNA Coding for it

[0045] (1) Because the sialidase of the present invention exhibits substrate specificity unique in that it substantially specifically hydrolyze gangliosides, a recombinant having the enzyme or DNA coding for the enzyme has much possibility for use as a reagent for saccharide chain studies.

[0046] (2) As one of the means for normalizing abnormality of this enzyme observed in cancer cells, for example, antisense therapy, which is a kind of gene therapy, will be expected in future. The gene structure clarified by the present invention is the important information for it. Moreover, if the expression mechanism of this enzyme becomes clear by genome structure analysis utilizing the cDNA as a probe, it will also become possible to normalize the abnormality of the expression of this enzyme in cancer and the like.

[0047] (3) Because of two reasons, i.e., the characteristic that the enzyme specifically decomposes gangliosides that are main components of brain, and its involvement in differentiation of nerve cells, the abnormalities of this enzyme may be found in certain brain diseases. In such a case, the information about the gene may be much utilized for development of gene therapy and medicaments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048]FIG. 1 represents the amino acid sequences of peptides obtained by endoproteinase digestion and lysyl endopeptidase digestion. The amino acids represented with minor characters in the figure are amino acids of low determinancy. A Lys residue presumed to have bound to the N-terminus amino acid is represented with (K).

[0049]FIG. 2 represents alignment of deduced amino acid sequences of a PCR product (BBmSD) obtained by using bovine brain cDNA as a template and a rat skeletal muscle cytoplasmic sialidase (RMcSD). Common amino acids are represented with dots “.”, and analogous amino acids are represented with asterisks “*”. The highly homologous regions used for the preparation of a probe are underlined.

BEST MODE FOR CARRYING OUT THE INVENTION

[0050] Hereafter, the present invention will be explained more specifically with reference to the following examples.

[0051] <1> Purification of Sialidase Localized in Plasma Membrane

[0052] (1) Method for Measuring Sialidase Activity

[0053] In this example, sialidase activity was measured as follows.

[0054] A reaction system (0.2 ml) contained 50-100 nmol of sialic acid-bound saccharide substrate, 0.2 mg of bovine serum albumin, 15 mmol of sodium acetate buffer (pH 4.6), 0.2 mg of Triton X-100, and an enzyme fraction, and a substrate consisting mainly of bovine brain mixed gangliosides (Sigma, Type II) was used.

[0055] A reaction mixture having the aforementioned composition was incubated at 37° C. for 15-60 minutes, and the reaction was stopped by quick freezing. The released sialic acid was quantitated by the thiobarbituric acid method of Warren (Warren L., J. Biol. Chem. 234, 1971-1975, 1959) at 549 nm and 532 nm. In Steps 1, 2, and 7 explained below, the quantitation was performed by the same method after the reaction product was passed through an AGX-2 ion exchange mini column. The amount of the sialic acid (nmol) released per hour was defined as 1 unit. When a synthetic substrate, 4-methylumbelliferyl N-acetylneuraminic acid (4MU-NeuAc) was used as the sialic acid-bound saccharide substrate, Triton X-100 was excluded from the reaction system, and released 4-methylumbelliferone was quantitated by a fluorospectrophotometer.

[0056] In this example, the amount of enzyme protein was measured by the Bradford method (a kit of Biorad Co. was used), or the BCA method (Piece Chemical Co.). Further details of the measuring methods are found in the previous report (Miyagi and Tsuiki, J. Biol. Chem. 260, 6710-6716, 1985).

[0057] (2) Solubilization and Purification of Bovine Membrane-Bound Sialidase

[0058] The whole procedure described below was carried out at 4° C. Bovine brains obtained from a slaughterhouse were freezed at −80° C. until they were used.

[0059] 200 g of bovine brain was added 9 volumes of 0.32 M sucrose, 1 mM DTT (dithiothreitol), 1 mM EDTA, and 0.1 mM PMSF (phenylmethylsulfonyl fluoride), homogenized by a glass Teflon homogenizer, and centrifuged at 1000×g for 10 minute. The resulting supernatant was further centrifuged at 30,000×g for 1 hour. After the centrifucation, the resulting precipitate fraction (Step 1) was suspended in Buffer A (20 mM potassium phosphate, pH 6.8, 0.1% Triton X-100, 1 mM EDTA, 1 mM DTT) containing 180 ml of 0.1 mM PMSF, then added 5% deoxycholic acid, sufficiently homogenized, and centrifuged at 100,000×g for one hour to obtain a supernatant as a soluble fraction (Step 2).

[0060] The soluble fraction was applied to a DEAE-cellulose column (4.5×20 cm) equilibrated with Buffer A, washed, and eluted with Buffer A containing 0.2 M NaCl to collect 15 ml-fractions (Step 3). The active fraction was dialyzed against Buffer A, then applied to an Octyl-Sepharose column (2.5×7 cm) equilibrated with the same buffer, and eluted with a linear gradient (400 ml) of 0.1-0.4% Triton X-100 to collect 10-ml fractions (Step 4).

[0061] Then, the active fraction was applied to a Heparin-Sepharose column (1.5×1 cm), washed with Buffer A containing 0.25 M NaCl, and eluted with a 0.2-1 M NaCl linear gradient (200 ml) in Buffer A, and the active fraction was concentrated by ultrafiltration using a YM-10 membrane (Step 5).

[0062] Concentrated enzyme solution obtained from 3 times of the elution from the Heparin-Sepharose column was loaded on a Sephacryl S-200 column (Pharmacia, 1.5×2.5 cm.), and eluted with Buffer B (20 mM potassium phosphate, pH 6.8, 0.04% Triton X-100, 1 mM EDTA, 1 mM DTT, 0.02 mM NeuAc2en [2-deoxy-2,3-dehydro-N-acetylneuraminic acid]) at a flow rate of 10 ml/h to collect 2-ml fractions (Step 6).

[0063] The active fraction from Step 6, which was diluted so that it should have a Triton X-100 concentration of 0.02%, was applied to an RCA-lectin agarose column (1.5×2.5 cm) equilibrated with Buffer B in which only the concentration of Triton X-100 was changed to 0.02%, washed with the same buffer, and eluted with Buffer B containing 0.2 M lactose (Step 7). This active fraction was applied to a MonoQ (HR 5/5) column (Pharmacia), eluted with a 0-0.5 M NaCl linear gradient in Buffer B, and stored at −20° C. (Step 8).

[0064] The fraction obtained from 3 times of the elution in Step 8 (corresponding to 1.8 kg of the starting material) was loaded on an activated thiol-Sepharose column (Pharmacia, 1.5×2 cm), and eluted with Buffer B containing 0.15 M NaCl and 10% glycerol, and then with Buffer B containing 0.5 M NaCl and 10% glycerol, and eluted with Buffer B containing 0.5 M NaCl and DTT at a concentration raised to 50 mM. The active fraction was concentrated with a MonoQ column as in Step 8 (Step 9).

[0065] Finally, affinity column chromatography utilizing a synthetic ganglioside GM3 [GSC-211, NeuAc-Gal-Glc-O(CH₂)₈NH₂] (Hasegawa A. et al., J. Carbohydr. Chem., 9, 201-214, 1990) as a ligand was performed. An affinity column (0.7×3 cm) was prepared by allowing the GSC-211 to couple to ECH-Sepharose (Pharmacia) in the presence of N-ethyl-N′-(3′-dimethyl-aminopropyl)carbodiimide hydrochloride according to the instruction of the manufacturer. The enzyme fraction obtained from Step 8 was loaded on the column equilibrated with Buffer C (10 mM potassium phosphate, pH 6.8, 0.04% Triton X-100, 1 mM EDTA, 1 mM DTT, 20% glycerol), and eluted with a 0-0.5 M NaCl concentration gradient in Buffer C to collect 1.5 ml-fractions (Step 10).

[0066] The purification process using 3.5 kg of bovine brains as the start material was summarized in Table 1. By the procedure explained above, the sialidase activity was purified by more than 100,000 times from the bovine brain particulate fraction. The final sample was subjected to SDS-polyacrylamide gel electrophoresis according to the method of Laemmli (Laemmli, U. K. Nature, 227, 680-685, 1970) to determine its purity. As a result, while a weak band at 50 k was observed other than the main 52 k protein band, the staining density of the 52 k protein was parallel with the activity elution pattern from the affinity column, and in addition, this band was concentrated from Step 9 to 10. Therefore, it was considered to be a sialidase enzyme protein. TABLE 1 Total amount of Total Specific Purification proteins activity activity degree Yield Step (mg) (Unit) (Unit/mg) (-fold) (%) Particulate 36622 1666327 45.5 1 100 fraction Solubilized 23057 1616337 70.1 1.5 97 fraction DEAE- 13440 1051740 78.3 1.7 63 cellulose Octyl- 1581 486000 307 6.7 29 cellulose Heparin- 112 245520 2188 48 15 Sepharose Sephacryl 4.51 44670 9926 218 2.6 S-200 RCA-lectin 0.521 25771 49464 1087 1.5 agarose Mono Q 0.220 19590 89045 1957 1.2 Activated- 0.0103 18660 1811650 39816 1.1 thiol Sepharose Ganglioside 0.0012 5773 4851260 106621 0.34 Sepharose

[0067] (3) Physicochemical Properties of Bovine Membrane-Bound Sialidase

[0068] The physicochemical properties of the enzyme investigated by using the aforementioned purified enzyme are shown below.

[0069] (i) Substrate Specificity

[0070] The results of investigations about the activity of the enzyme of the present invention for various substrates are shown in Table 2. The numerical values represent relative activity when the activity for the GD3-ganglioside is defined to be 100. TABLE 2 Substrate Hydrolysis activity (%) Ganglioside GD3-ganglioside 100 GD1a-ganglioside 56 GM3-ganglioside 62 GM2-ganglioside 3 GM1-ganglioside 1 Synthetic ganglioside GSC-17 (α2-3) 110 GSC-61 (α2-6) 44 Orosomucoid 2 Fetuin 2 Glycophorin 3 Ovine submaxillary gland mucin 3 Bovine submaxillary gland mucin 0 α2-3 syalyllactose 11 4-MUNeuAc 25

[0071] The hydrolysis of GSC-17 (α2-3) occurred at a rate 2.5 times higher than that of the hydrolysys of GSC-61 (α2-6), and hence the enzyme is considered to be more likely to act on α2-3 linkage compared with α2-6 linkage. Further, since it did not act on α2-3 syalyllactose corresponding to the saccharide segment of the GM3 ganglioside, the ceramide segment is indispensable for a substrate.

[0072] (ii) Optimum pH

[0073] 4.7 to 5.0

[0074] (iii) Molecular Weight

[0075] About 65,000 as determined by sucrose density gradient centrifugation,

[0076] About 52,000 as determined by SDS-polyacrylamide gel electrophoresis under reducing condition.

[0077] (iv) Inhibition, Activation Etc.

[0078] A detergent is required for the activity. For example, it is highly active in the presence of 0.1 to 0.2% of Triton X-100.

[0079] Residual activities in the presence of various inhibitors are shown in Table 3. The numerical values each represent 100 minus (residual activity (%) in the presence of inhibitor). TABLE 3 Inhibitor Inhibition (%) CuCl₂ (1 mM) 95 4-Hydroxy mercury benzoate (50 μM) 92 2-Deoxy-2,3-dehydro-N- 45 acetylneuraminic acid (0.2 mM)

[0080] (3) Peptide Sequencing

[0081] Because the above-obtained final product was obtained at a low yield, the enzyme fraction of Step 9 was prepared in a similar manner by using 6 kg of bovine brains as the starting material, and subjected to peptide analysis. The enzyme fraction was desalted by dialysis in the presence of 0.1% PVP-40 (Sigma), concentrated with Centricon (Millipore), subjected to SDS-polyacrylamide gel electrophoresis as described above, and transferred to a PVDF membrane (Problott, Applied Biosystems). The location of the enzymatic protein was confirmed by Ponceau S staining, and the corresponding part of the membrane was excised, and digested with lysyl endopeptidase and then with endoproteinase Asp-N. The product was separated by high performance liquid chromatography.

[0082] The fractionated peptide was subjected to amino acid sequencing using a peptide sequencer (Shimazu PSQ-1). The above microsequencing was performed according to the method of Iwamatsu et al. (Iwamatsu A. and Yoshida-Kubomura N., J. Biochem. 120, 29-34, 1996). The obtained sequences are represented in FIG. 1 and SEQ ID NOS: 5-9. SEQ ID NOS: 5-7 are amino acid sequences of the fragments obtained by the endoproteinase digestion, and SEQ ID NOS: 8 and 9 are amino acid sequences of the fragments obtained by the lysyl endopeptidase digestion. In SEQ ID NO: 5, the 2-5th amino acids are indefinite, and it is highly possible that the 2nd amino acid should be Ala or Arg, the 3rd amino acid be Glu or Gly, the 4th amino acid be Ile or Tyr, and the 5th amino acid be Leu or Ser.

[0083] <2> cDNA Cloning of Bovine Brain Sialidase

[0084] Based on the amino acid sequence of the peptide of the purified enzyme, which had been determined as described above, 10 sense or antisense degenerate primers of SEQ ID NOS: 10 (DN1-1S), 11 (DN1-1A), 12 (DN1-2S), 13 (DN1-2A), 14 (DN2S), 15 (DN2A), 16 (DN3A), 17 (AP1A), 18 (AP3S), and 19 (AP3A) were prepared (see FIG. 1). DN1, DN2, DN-3, AP-1, and AP-3 are designations of the peptides shown in FIG. 1, S means “sense”, and A means “antisense”. DN-1 represents a nucleotide sequence determined by assuming that the undefinite amino acids of DN-1 (the 2-5th amino acids) should be Ala, Glu, Ile, and Leu, respectively, and DN-2 represents a nucleotide sequence determined by assuming that the undefinite amino acids of DN-1 should be Arg, Gly, Tyr, and Ser, respectively.

[0085] Bovine brain total RNA was prepared by the acid guanidium-phenol-chloroform method (Chomczynski P. and Sacchi N., Anal. Biochem. 162, 156-159, 1987), and poly(A)⁺RNA was purified by oligo(dT)-cellulose column chromatography. cDNA was prepared in accordance with the previous report (Miyagi T. et al., J. Biol. Chem., 268, 26435-26440, 1993) using the poly(A)⁺RNA (1 mg) and reverse transcriptase (derived from Molony murine leukemia virus, BRL), and amplification by PCR utilizing the cDNA as a template was attempted.

[0086] The PCR reaction mixture (50 ml) had a composition of 50 mM KCl, 10 mM Tris-HCl (pH 8.3), 1.5 MM MgCl₂, 0.01% of gelatin, 0.2 mM dNTPs (2 mM each of dATP, dGTP, dCTP and dTTP), 0.5 mg of cDNA, and 1.5 units of Taq polymerase (Ex Taq, Takara). The DNA amplification was performed by 40 cycles of reactions of at 94° C. (0.5 minutes), 50° C. (1 minute) and 72° C. (2 minutes), followed by extension reaction at 72° C. for 10 minutes. The obtained 12 DNA amplification fragments were each subcloned in the SmaI site of Bluescript vector (Stratagene), and subjected to DNA sequencing by the dideoxy method (Sanger F. et al., Proc. Natl. Acad. Sci. USA, 74, 5463-5467, 1977).

[0087] As a result of examination for the validity of the amino acid sequences deduced from the nucleotide sequences of the segments corresponding to the primers within the amplified DNA fragments, presence or absence of a stop codon and the like, it was found that only a PCR product of 0.5 kb obtained with primers AP3S and DN2A fulfilled those requirements. In addition, two Asp-boxes (-Ser-Xaa-Asp-Xaa-Gly-Xaa-Thr-Trp- (SEQ ID NO: 20)), which is a consensus sequence of sialidase, were found in this fragment (amino acid numbers 131-138 and 205-212 in SEQ ID NO: 2), and the deduced amino acid sequence exhibited 38% homology with respect to the amino acid sequence of the cytoplasmic sialidase, which we had previously isolated. However, it did not exhibit significant homology with any other proteins.

[0088] Then, a bovine brain λgt10 library (Clontech) was screened by using the above 0.5 kb cDNA as a probe. The cDNA was isotope-labeled with [α-³²P]dCTP using Random Primer DNA Labeling Kit (Takara), and phages (2×10⁶) were screened by plaque hybridization. The hybridization was carried out by using a nylon membrane (Hybond N⁺, Amersham) according to the instruction of the manufacturer. Among 15 positive clones, two of pBB121 (1.45 kb) and pBB321 (2.8 kb) were estimated to contain the full coding region.

[0089] The nucleotide sequence of the insert of pBB321 (2.8 kb) and the amino acid sequence deduced from it are shown as SEQ ID NOS: 1 and 2. It was found that four types of amino acid sequences obtained from the peptide of the purified product were contained in it, and as for DN-1, the 2nd amino acid of DN 1-1 was A R. Moreover, because it was found that the same sequence was contained in bovine keratin based on protein database search, although it was not found for the AP-1 sequence, it is highly possible that it originated from contamination of keratin in the enzyme fraction used for the peptide sequencing.

[0090] In addition to the above-mentioned two Asp-boxes, another Asp-box was further found on their 3′ side (the amino acid numbers 256-263 in SEQ ID NO: 2). A hydrophobic sequence considered to be a transmembrane domain was found between the two Asp-boxes (the amino acid numbers 174-194 in SEQ ID NO: 2), and a glycosylation site was found on its 3′ side (the amino acid number 349 in SEQ ID NO: 2). Because the enzyme has the characteristic that it is bound to RCA lectin as used in the purification procedure, it is considered that a saccharide chain actually attached to this site. The molecular weight of the protein calculated from the 428 amino acids is 48,000, and if one saccharide chain is attached, the actual value will become around 50,000, and it is not contradictory to the value determined for the above purified product by SDS-polyacrylamide gel electrophoresis.

[0091] <3> Transient Expression of Sialidase cDNA in COS Cell

[0092] The coding region of pBB121 (1.45 kb) was amplified by PCR using 5′ sense primer (SEQ ID NO: 21) which was added an EcoRI site and 3′ antisense primer (SEQ ID NO: 22), and the obtained DNA fragment was purified by agarose electrophoresis. This product was ligated to the EcoRI site of SRα promoter high expression vector pME18S (provided by Dr. Kazuo Maruyama, Medical Department, Tokyo Medical and Dental University) having SV40 replication origin (pME18S-mSD), and introduced into COS-7 cells by electroporation to attempt its transient expression. Forty μg of pME18S or pME18S-mSD was added to COS-7 cells (10⁶) cultured in DMEM containing 10% FBS (fetal bovine serum) in the logarithmic growth phase, left at room temperature for 10 minutes, applied with electric pulses at 250 V and 950 μFD, left at room temperature again for 10 minutes, and then returned to the culture.

[0093] The cells after the culture of 48 hours were collected. After the blood serum components were removed with PBS, the cells were added 9 volumes of PBS, and disrupted by ultrasonication for 10 seconds. The disrupted cell suspension was centrifuged under cooling at 1,000×g for 10 minutes, and the resulting supernatant was used as a homogenate. Sialidase activity in the homogenate of the transfectants was measured by using gangliosides as the substrate in the presence of Triton X-100 (0.1%).

[0094] The specific activity of control cells having only the vector and the cells that were introduced with pME18S-mSD was 23.4 units/mg protein and 844.5 units/mg protein, respectively. Thus, the cells introduced with pME18S-mSD exhibited the activity 36 times higher than that of the control cells. However, increase of the activity for hydrolyzing 4MU-sialic acid was not observed at all. This result confirmed the results of the previous characterization of the purified product of the bovine brain enzyme, i.e., the expressed sialidase substantially specifically acted on gangliosides, and hardly acted on synthetic substrates such as 4MU-sialic acid.

[0095] Further, it was investigated whether the expressed sialidase localized in plasma membrane or not by using Percoll (Pharmacia) concentration gradient centrifugation. In accordance with a previous report (Sagawa J. et al., J. Biochem. 107, 452-456, 1990), the homogenate was overlaid on 40% Percoll containing 0.25 M sucrose, centrifuged at 48,000×g for 40 minutes, and fractionated, and the sialidase activity was measured. Ganglioside sialidase activity was detected at the same location as the activity distribution of 5′-nucleotidase or alkali phosphatase, which are marker enzymes of plasma membrane, and thus the localization of the expressed sialidase in plasma membrane was confirmed.

[0096] <4> cDNA Cloning of Human-Derived Ganglioside Sialidase

[0097] When the primary structure of the bovine brain sialidase was compared with the previously isolated cytoplasmic sialidase (Miyagi T. et al., J. Biol. Chem., 268, 26435-26440, 1993), it was found that they contained a sequence well conserved in them (FIG. 2). Therefore, one set of primers was prepared based on this amino acid sequence (SEQ ID NOS: 23 and 24). In the amino acid sequences shown in FIG. 2, the partial sequence of cDNA for bovine brain sialidase (BBmSD) corresponds to the amino acid numbers 49-209 of SEQ ID NO: 2. The rat skeletal muscle cytoplasmic sialidase (RMcSD) corresponds to the amino acid numbers 1-240 in the amino acid sequence of the sialidase.

[0098] Human brain cDNA and human kidney cDNA were prepared under the same condition as the case of the bovine enzyme, and PCR was performed by using them as the template. The amplified DNA fragment of 0.25 kb was subcloned, and the DNA sequence was determined. One Asp-box was found in this cDNA. A human brain λgt10 cDNA library and human kidney λgt10 cDNA library (Clontech) were screened by using the above DNA as a probe. By screening 8×10⁵ plaques for human brain and 1×10⁶ plaques for human kidney, three positive clones (pHB82, pHB85, and pHB95) and one positive clone (pHK65) were obtained, respectively.

[0099] When these DNA sequences were investigated after subcloning, an overlapped portion of 1 kb was found in all of them. Nucleotide sequences obtained from pHB95 containing the substantially whole coding region and pHK65 containing 3′ end non-coding region of 1 kb, and deduced amino acid sequences therefor are shown as SEQ ID NOS: 3 and 4. They exhibited high homology with the sequences for bovine brain enzyme, i.e., 81% (87% for only the coding region) on the nucleotide level, and 82% on the amino acid level.

[0100] In SEQ ID NO: 4, the transmembrane domain corresponds to the amino acid numbers 174-194, the glycosylation site to the amino acid number 348, and the Asp-boxes to the amino acid numbers 131-138, 205-212, and 256-263.

[0101] When the expression status was investigated in various human tissues by Northern blotting using the 1.5 kb insert of pHB95 as the probe, high expression of mRNA of about 4 kb was observed in skeletal muscles, and mRNA of the same size was also detected in brain, liver and the like.

INDUSTRIAL AVAILABILITY

[0102] The present invention provides sialidase localized in plasma membrane and DNA coding for it. The sialidase of the present invention differs from sialidases known so far in that it mainly localizes in plasma membrane, and specifically hydrolyze gangliosides. 

What is claimed is:
 1. A protein defined in the following (A) or (B): (A) a protein which has the amino acid sequence of SEQ ID NO: 2 or SEQ ID NO: 4, or (B) a protein which has the amino acid sequence including substitution, deletion, insertion, or transition of one or several amino acid residues in SEQ ID NO: 2 or SEQ ID NO: 4, and exhibits activity to eliminate a sialic acid residue from a non-reducing terminal of ganglioside.
 2. A DNA coding for a protein defined in the following (A) or (B): (A) a protein which has the amino acid sequence of SEQ ID NO: 2 or SEQ ID NO: 4, or (B) a protein which has the amino acid sequence including substitution, deletion, insertion, or transition of one or several amino acid residues in SEQ ID NO: 2 or SEQ ID NO: 4, and exhibits activity to eliminate a sialic acid residue from a non-reducing terminal of ganglioside.
 3. The DNA according to claim 2 which has the nucleotide sequence of SEQ ID NO: 1 or SEQ ID NO:
 3. 